Electroencephalography (EEG) is one method to monitor electrical activity of the brain. It is typically noninvasive, with the electrodes placed along the scalp, however, invasive electrodes may be used in specific applications. EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain. However, the sensitivity of the EEG electrodes limits detection to small regions of the brain, close to each electrode, thus limiting the spatial resolution of EEG.
Functional magnetic resonance imaging (fMRI) is another method to monitor activity of the brain. However, a magnetic resonance imager is a large and expensive clinical device which can neither be used outside of the clinic, nor in a continuous manner.